In a high contrast projector, a light source illuminates a “premod” DMD (e.g. a premodification digital micromirror device), which modulates light from the light source to form an image, which is in turn used to illuminate a prime DMD premod, bitplanes (images) at the premod DMD can be used to achieve selective illumination of the prime DMD, reducing black levels and increasing contrast. To allow faster switching of images, groups of DMD pixels can be updated instead of the entire DMD image, reducing the time to load the image data as only a portion of the DMD must be loaded with the image data. Switching a group of pixels (e.g. not the entire DMD) at a time is referred to as a “phased switch”, whereas switching the entire DMD to a new image is referred to as a “global switch”.
A group of pixels can be switched twice in rapid succession to display a bitplane for a short period of time (e.g. as short as the time required to load image data into the pixel group); the bitplane displayed briefly is referred to as a “phased bitplane”. However, when phased switching of bitplanes is used, artifacts can be introduced at pixel group boundaries due to incorrect illumination of prime DMD pixels in two ways: first, by the PSF (point spread function) spreading the image from premod DMD pixel groups configured for two different bitplanes across prime DMD pixels configured for a single bitplane; and second, by the PSF spreading the image from the premod DMD pixels, configured for a single bitplane, across prime DMD pixels configured for two different bitplanes. Specifically, a prime DMD pixel that is near the image of the premod pixel group boundary can be incorrectly illuminated by the PSF image from the premod DMD as the premod pixel groups on either side of the premod pixel group boundary switches from a first bitplane to a second bitplane in a phased manner. For DMDs, where the pixels can be grouped in 15 to 16 horizontal bands, the artifacts can be clustered around 14 or 15 horizontal lines in a projected image which can be noticeable to the human visual system.